Method of heating oil



Patented Dec. 3, 1929 PATENT OFFICE JOHN PRIMROSE, F RICHMOND, NEW IJRK, ASSIGNOR T0 FOSTER WHEELER CORPO- RATION, OF NEW YORK, N. Y., A. CORPORATION" OF NEW YORK METHOD OF HEATING OIL I Application filed July 2,

The general object of the present invention is to provide an improved method of and apparatus for heating oil especially devised and adapted for use in the so-called cracking processes of refining petroleum. More spear. cifically, the object of the invention is to effect the combustion of the fuel burned to heat the oil in such manner as to provide a relatlvely large weight of heating gases per pound of fuel burned with a relatively low maximum heating gas temperature, while at the same time maintaining a satisfactory thermal efficiency.

In ordinary oil cracking processes it is usually desirable to heat the oil to a temperature of something like 700 or 800 F. the exact temperature depending somewhat on the character of the oil treated and on the particular process followed. A desirable form of apparatus for heating the oil to the proper cracking temperature comprises a multiplicity 'of tubular heater or retort pipe elements which are mounted in a suitable furnace or retort housing and through which the oil treated passes and is progressively heated. In approved modern forms of such retort furnaces, a considerable portion of the heat generated in the furnace may be transmitted to the oil heater elements or retort proper by direct rao diation from the flames in the retort furnace chamber and from the refractory inner walls of the retort housing, but in all cases, a large portion of the heat absorbed must be transmitted to the heater elements or retorts prope1, by contact with the heating gases sweepwhich. is low enough from the standpoint of the thermal efficiency of the furnace, requires a path of heating gas flow in contact with the oil heater elements or retorts which is of appreciable length, and in which the heating gases progressively decrease in temperature 1923. Serial NO. 648,974.

along this path. The portions of the oil retort heating surface in different parts of the retort furnace are therefore exposed to very different temperatures, varying from the initial temperature of combustion down to the exit temperature of the heatin gases, which in practice is ordinarily somevf hat above the cracking temperature.

In the operation of oil cracking retorts, overheating of any portion of the retorts proper, is very objectionable, since in an oil cracking process, very complex chemical r actions occur, and the nature of these reactions depend largely on the temperature at which cracking occurs, though it may he modified somewhat by the pressure of the oil. In general when petroleum is cracked, there are formed a heavy oil residue, free carbon, permanent gases, and condensable vapors. The condensable vapors are the valuable products of the process, and it is to obtain those products that the process is carried on. The effect of increasing the cracking temperature above that desired and required, is to decrease the amount of valuable condensable products recovered, and to retort walls reduces the heat transfer capacity of the apparatus, and what is much more ima given weight ofthe fuel, and to corresponddanger of local overheating portions of the.

retortheating surface subjected to the higher temperatures, and desirably increases the effectiveness of the rtion of the retort heating surfaces absor ing heat by contact from the heatin gases, since the heat absorbing capacity 0 that surface depends on the ve-- locity of heating gas flow past it. The increased weight and lowered temperature of the heating gases also makes the heat absorption by contact more uniform along the length of the gas path of flow in which this contact occurs. In other words, it makes the heat absorption per unit of retort heating surface adjacent-the exit point-of the heating gases substantially nearer a parity with the heat absorption per unit of retort heating surface at the entrance end of that path.

Increasing the amount and reducing the initlal temperature of the heatin gases as described, substantially increases t e amount of heat carried out of the retort housing proper, by the heating gases, and wasted, if these gases pass directlyto the stack, but in accordance with the present invention, I minimize this disadvantage by utilizing the available heat in the heating gases leaving th s retort furnace to preheat the air supplied to the retort furnace for the combustion of the fuel, and for the production of the de-- sired quantity of heating gases.

In the accompan ing drawings I have diagrammatically i lustrated a preferred form of apparatus for. carrying out my invention.

Of the. drawings:

Fig. 1 is a plan; and

Fig. 2'is an elevation partly in section.

' The apparatus shown in the drawings comprises a unit consisting of two retort furnaces A, and a stack B. 'Each furnace A, as illustrated, comprises two chambers A I and A separated by a bridge wall A. The chamber A is the combus ion chamber into which the fuel burned, and the air for its combustion are introduced, As shown the furnace is heated by the combustion of oil supplied through the pipe C, though it will be understood that the furnace may be gas or coal fired. D represents the air inlet to the chamber A. The oil heaters or retorts proper comprise tubular elements E, E and E The elements E are located in the side walls of the chamber A, and the elements E are located in the roof over the chambers A" and A, and these elements are intended to absorb heat mainly by directradiation, and are constructed in known manner to adapt them for this :The elements E constitute a bank of tubes through which the heating gases pass to the retort furnace outlet F. The outlet F of each furnace is directly connected by a corresponding flue F to the stack B. Each flue F is provided with a normally closed damper G located between branches F and F of the flue F leading to the inlet and outlet, respectively of an air heater H. The latter, in the particular construction diagrammatically illustrated, comprises tubes H into which the heating gases pass from the flue F through the branch F and from which they pass back to the flue F through the branch F Ordinarily I consider it desirable to supplement the stack suction on each air heater outlet by interposing an exhaust fan I between the heater outlet and the branch flue F proper. Each air heater H is provided with an inlet H through which atmospheric air passes into the intertube space of the heater, and an outlet from this space through which air is conducted by a flue or conduit D to the furnace air inlet D.

and air supplied can obviously be regulated by suitable adjustment of theoil regulating provisions such as thesupply valve C, and by regulating the draft, as by regulating the speed of the corresponding exhaust fan.

The following comparison of results obtainable with and. without the use of the invention illustrates characteristic advantages of the invention. Assume for example,

that an oil heating furnace is to absorb ten million B. t. u.s' per hour that the fuel is oil giving 18,500 E. t. u.s per pound, that the heatinggases'leave the furnace pro or at a temperature of 850 F., that the speclfic heat of the gases is .25, that the efficiency of heat transfer from the heating gases, to the retorts is .95, and that the furnace efliciency is .90. If with the assumptions made, the maximum furnace temperature is 2500 R, which when the air for combustion is not preheated would be as low as would ordinarily be considered desirable from the standpoint of overall efliciency, each pound of heating gases would supply 392 B. t.'u.s 2500- 850) .25 .95=392). With each pound of heating gases furnishing 392 B. t. u.s, 25500 lbs. of heating gases would be required to furnish ten million B. t. u.s, and each pound usages were supplied to the furnace at a temperature Under these circumstances the oil required to furnish ten million B. t; u.s would be 925 lbs.

27.5 4 This would give an overall thermal efliciency With the oil supplied at 60 F., and the air supplied at 400 F., each pound of oil burned would give about 40.8 pounds of oil vapor and air at a temperature of 2000 F. (408x (2000 100) .25+1 (400- 60) =18500 .90, approximately). There would thus be required about 897 pounds of fuel oil per hour, and the overall thermal efficiency would be about 60.2%.

The use of the invention under the conditions assumed above therefore increases the overall thermal efiiciency, while at the same time permitting a reduction in maximum furnace temperature from 2500 F. to 2000 F. This reduction in maximum furnacechamher, as already explained is highlydesirable, and amounts to a very substantial improvement in operating efiiciency, as it tends to a better yield of the valuable condensable products, and reduces the liability of trouble from retort overheating and carbon deposits.

Theparticular form of furnace illustrated comprises novel features of combustion and arrangement devised by me but not claimed herein as they are claimed in my. copending application, Serial No. 611,932, filed Jan. 11, 1923. While this furnace is well adapted for use in carrying out the present invention, those skilled in the art will understand that the present invention may be used with many difl'erenttypes and styles of furnaces.

Having now described my invention, what I claim is new and desire to secure by Letters Patent, is:

The method of operating an oil still which j consists in burning fuel in an enclosed space,

supplying air-in substantial excess of the amount required for the combustion of the fuel to substantially lower the temperature in said space and to substantially increase the volume of combustion gases, absorbing heat by direct radiation from said space and transferring it to oil circulating in said s ace, passing the products of combustion an the excess air mixed therewith into a second enclosed space, cooling said gases during their passa e to said second space by transferring heat y convection to circulating oil, and utilizing the remaining heat in said gases for heating oil circulating in said lastmentioned space and preheating substantially all of the excess air supplied to said first enclosed space before its entrance thereto.

Signed at New York in the county of New Yorkand State of New York this 29th day of June A. D. 1923.

JOHN PRIMIROSE. 

